In recent years, with the spread of various compact-type mobile electronic devices such as a mobile electronic terminal represented by a mobile phone, a notebook-type PC or the like, a secondary battery has fulfilled an important role as a power source thereof.
A lithium secondary battery is widely used as a power source of electronic devices of, for example, a mobile phone, a notebook-type PC or the like, a power source for an electric automobile or a power storage, or the like, and is mainly composed of a positive electrode, a non-aqueous electrolytic solution and a negative electrode.
As the positive electrode composing a lithium secondary battery, for example, LiCoO2, LiMn2O4, LiNiO2, LiFePO4 or the like has been known. It has been reported that, in the lithium secondary battery using these, when it becomes high temperature in a charged state, since a non-aqueous solvent in the non-aqueous electrolytic solution is locally subjected to oxidative decomposition partially at the interface between a positive electrode material and the non-aqueous electrolytic solution, a decomposed substance or gas generated by this decomposition inhibits an original electrochemical reaction of a battery, and as a result, battery performance such as cycle characteristics is decreased.
In addition, as the negative electrode, for example, metallic lithium, a metallic compound being able to store and discharge lithium (for example, a metal elementary substance, an oxide, an alloy with lithium or the like), a carbon material, or the like has been known, and particularly, a lithium secondary battery using a carbon material such as coke, artificial graphite, natural graphite, which is able to store and discharge lithium, has practically been used widely.
It has been reported that the lithium secondary battery using a highly crystallized carbon material of, for example, artificial graphite, natural graphite or the like, as a negative electrode material, decreases cycle characteristics, because a non-aqueous solvent in the non-aqueous electrolytic solution is reductively decomposed at a negative electrode surface in charging, and a decomposed substance or gas generated thereby inhibits an original electrochemical reaction of a battery. In addition, it has been known that the lithium secondary battery using, for example, a lithium metal or an alloy thereof, a metal elementary substance or an oxide using tin, silicon or the like, as a negative electrode material, has high initial capacity, however, because the negative electrode material becomes fine powder in cycle, reductive decomposition of the non-aqueous solvent occurs at an accelerated rate, as compared with a negative electrode made of the carbon material, resulting in decrease in charge-discharge efficiency at the first cycle accompanied with increase in initial irreversible capacity of a battery, and large decrease in battery performance such as battery capacity or cycle characteristics, accompanying therewith.
In this way, generation of fine powder of the negative electrode material or accumulation of a decomposed substance of the non-aqueous solvent inhibits smooth storage and discharge of lithium to the negative electrode, and as a result, has a problem of significant decrease in battery characteristics such as cycle characteristics.
As described above, a usual lithium secondary battery had a cause of decreasing battery performance, by inhibiting transfer of a lithium ion, or blistering the battery, by a decomposed substance or gas generated in decomposition of the non-aqueous electrolytic solution on the positive electrode or the negative electrode.
On the other hand, a tendency for multi-functionalization of electronic devices mounted with a lithium secondary battery has been progressing more and more, and it is now in a tendency of increasing power consumption amount. Accompanied with it, change to higher capacity of a lithium secondary battery has also been progressing more and more, and it has been a problem that volume occupied by the non-aqueous electrolytic solution inside a battery becomes smaller, for example, by improvement of increasing density of an electrode, decreasing useless space volume and dead space inside a battery or the like, and thus decomposition of a small amount of the non-aqueous electrolytic solution influences largely on decrease in battery performance.
Still more, in recent years, as new power source for an electric automobile or a hybrid electric automobile, there has been performed development of an electrical storage device; called an electric double layer capacitor using an activated carbon or the like for an electrode, in view of output density; called a hybrid capacitor (which utilizes both capacity by storage and discharge of lithium, and electric double layer capacity) combining principle of electricity accumulation of a lithium ion secondary battery and the electric double layer capacitor, in view of satisfying both energy density and output density, and it is a present state that enhancement of cycle characteristics or the like is required.
To enhance characteristics of the non-aqueous electrolytic solution battery, it has been required to enhance not only characteristics of a negative electrode or a positive electrode, but also characteristics of the non-aqueous electrolytic solution which takes a role of transfer of a lithium ion.
As the non-aqueous electrolytic solution of the non-aqueous electrolytic solution-type secondary battery at present, a non-aqueous solution is used, where a lithium salt (an electrolyte salt) of, for example, LiBF4, LiPF6, LiClO4, LiN(SO2CF3)2, LiN(SO2CF2CF3)2 or the like, is mixed into a non-proton organic solvent.
The non-aqueous electrolytic solution, where for example, LiBF4, LiPF6 or the like is dissolved into a non-aqueous solvent, has been known to be stable in high voltage, because of, for example, having high electrical conductivity, which exhibits transfer of the lithium ion, and high oxidative decomposition voltage of LiBF4 or LiPF6. Therefore, such the non-aqueous electrolytic solution-type secondary battery contributes to bring out features of having high voltage and high energy density (PATENT LITERATURE 1).
However, the non-aqueous electrolytic solution composed of a non-aqueous solvent dissolved with LiBF4 or LiPF6 as a lithium salt, has a problem of generation of hydrogen fluoride (HF) caused by decomposition of the lithium salt at high temperature environment of 60° C. or higher, due to inferior thermal stability of these electrolytes. This hydrogen fluoride causes a phenomenon of decomposition of, for example, a carbon material of a negative electrode in a battery or the like, therefore had a problem of not only decrease in battery capacity caused by decrease in charge-discharge efficiency at the first cycle or the like, accompanied with increase in initial irreversible capacity of a secondary battery provided with such the non-aqueous electrolytic solution, but also increase in internal resistance of a battery under high temperature environment, and significant decrease in battery performance such as charge-discharge cycle life.
In addition, as the non-proton organic solvent to dissolve the lithium salt in the non-aqueous electrolytic solution, for example, carbonates such as ethylene carbonate, propylene carbonate, and dimethyl carbonate are mainly used, and among them, a mixed solvent combining a high dielectric constant solvent having high solubility of an electrolyte, and a low viscosity solvent is preferable. It is because the high dielectric constant solvent has high viscosity and very slow ion transportation, and is thus required to decrease viscosity thereof to increase ion transportation and increase ionic conductivity. Specifically a mixed solvent composed of a cyclic carbonate ester of, for example, ethylene carbonate, propylene carbonate or the like, as the high dielectric constant solvent, and a straight chained carbonate ester of, for example, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate or the like, as the low viscosity solvent, is used, and an electrolytic solution comprising this provides high ionic conductivity.
However, in the case of using the mixed solvent composed of the cyclic carbonate ester such as ethylene carbonate, and the straight chained carbonate ester such as dimethyl carbonate or ethylmethyl carbonate, an ester exchange reaction brings about on the electrode, by the straight chained carbonate ester, which generates an alkoxide radical such as a methoxy group or an ethoxy group, as an intermediate thereof. Since these radicals generated by this ester exchange are strong nucleophilic agents, they promote ring-opening/decomposition of ethylene carbonate, propylene carbonate or the like, which is a cyclic carbonate ester, and generate gas, or dissolve a metal of the positive electrode active material and destroy a crystal structure, and as a result, it has a problem of increasing initial irreversible capacity of a battery, and decreasing battery characteristics such as battery capacity or cycle characteristics accompanying therewith.
For example, at the graphite negative electrode, potential for a lithium ion to be inserted is at the vicinity of 0.3 V (vs. Li+/Li), and at this potential, most of organic solvents are decomposed. Therefore, a reductive decomposition reaction of an electrolytic solution using ethylene carbonate or the like is generated at the vicinity of 1.0 V, in initial charging. Because a decomposed product of the electrolytic solution forms a surface coating on the graphite negative electrode, and suppresses the reductive decomposition of the electrolytic solution at or subsequent to the second cycle, charge-discharge efficiency at or subsequent to the second cycle becomes nearly 100%. However, because of generation of irreversible capacity caused by formation of the surface coating in the initial charging, research and development aiming at decreasing the above irreversible capacity of a battery by optimizing the electrolytic solution has been competed (NON-PATENT LITERATURE 1).
As described above, the non-aqueous electrolytic solution-type secondary battery has a problem of decrease in charge-discharge efficiency at the first cycle accompanied with increase in initial irreversible capacity of a battery, as well as a problem of decrease in electric capacity or increase in internal resistance, by storage at high temperature or repeated charge-discharge, and thus various additives have been proposed to enhance stability or various battery characteristics of the non-aqueous electrolytic solution-type secondary battery.
For example, in a secondary battery using a negative electrode made of graphite having high crystallinity, there have been proposed, for example, a non-aqueous electrolytic solution containing vinylene carbonate, vinylethylene carbonate or the like (PATENT LITERATURE 2 and PATENT LITERATURE 3), a non-aqueous electrolytic solution containing 1,3-propanesultone and butanesultone, for example, in a secondary battery using a carbon negative electrode (PATENT LITERATURE 4).
Because an electrolytic solution containing a cyclic carbonate compound having an unsaturated group such as vinylene carbonate, vinylethylene carbonate, or a sultone compound such as 1,3-propanesultone or butanesultone, forms a stable coating which suppresses reductive decomposition of the electrolytic solution at the negative electrode surface, by polymerization/reductive decomposition of these additives at the negative electrode surface, a side reaction such as decomposition of a solvent, which has been occurred at the negative electrode surface, can be suppressed by covering the negative electrode surface with this reactive coating layer, resulting in improvement of a problem of decrease or the like in charge-discharge efficiency at the first cycle, accompanied with increase in initial irreversible capacity of a battery. Therefore, the electrolytic solution containing these additives provides certain degree of effect, even in the case of using any negative electrode, however, in particular, for a negative electrode made of highly crystalline natural graphite or artificial graphite, vinylene carbonate exhibits effect of peeling suppression of a graphite layer, therefore it has been used widely as additives for the electrolytic solution of a battery having these as a negative electrode.
On the other hand, additives forming a coating on a negative electrode have been reported, other than the above sultone compound or the cyclic carbonate compound having an unsaturated group. There are included an electrolytic solution containing, as additives, a disulfonate ester derivative such as, for example, propylene glycol dimethanesulfonate, 1,4-butanediol dimethanesulfonate (PATENT LITERATURE 5, PATENT LITERATURE 6, and PATENT LITERATURE 7); an electrolytic solution containing, for example, both a disulfonate ester derivative such as ethylene glycol dimethanesulfonate and a sulfonate ester derivative such as methyl methanesulfonate (PATENT LITERATURE 8 and PATENT LITERATURE 9); an electrolytic solution containing, for example, a fluorine-containing sulfonate compound (PATENT LITERATURE 10 and PATENT LITERATURE 11); or the like.
However, for example, the above disulfonate ester derivative, the sulfonate ester derivative, the fluorine-containing sulfonate compound or the like have not sufficient coating forming ability on the negative electrode, and has a problem of not forming a coating sufficient to suppress reductive decomposition of the non-aqueous electrolytic solution, as well as has not sufficient durability of the coating. As a result, initial irreversible capacity increases, to generate a problem of decrease in charge-discharge efficiency at the first cycle. Even by adding the disulfonate ester derivative in excess into the non-aqueous electrolytic solution to improve this point, resistance of a coating component generated at the negative electrode surface increases, which, to the contrary, raises a problem of leading to decrease in battery performance. Therefore, the addition of these additives to the electrolytic solution was not sufficient to enhance total balance of battery characteristics and cost of the non-aqueous electrolytic solution, as well as environmental aspect, production step or the like.
In addition, in preparing the non-aqueous electrolytic solution, there was a problem that a lithium salt in the non-aqueous electrolytic solution reacts with moisture inside a system and decomposes to generate a free acid such as hydrogen fluoride (HF), because of temperature increase of the non-aqueous electrolytic solution itself due to heat generation in dissolving and concocting the above lithium salt. In particular, in the case of concocting the non-aqueous electrolytic solution containing the above sultone compound or the disulfonate ester derivative or the like, there was a problem of increase in a free acid in the non-aqueous electrolytic solution, as a result of promotion of the above side reaction by increase in temperature in concocting, or decomposition or the like of the sultone compound or the disulfonate ester derivative or the fluorine-containing sulfonate compound itself, and thus it was necessary to prevent temperature increase of the non-aqueous electrolytic solution, and prevent deterioration of the non-aqueous electrolytic solution.
Under these circumstances, in conventional technology, investigation has been still performed on the non-aqueous electrolytic solution satisfying both the problem relating to the above lithium salt, and the problem relating to the non-aqueous solvent, and further improvement is necessary on a preferable combination that makes difficult to appear the effect of respective negative side of composition element of the non-aqueous electrolytic solution (a non-aqueous solvent, a lithium salt, additives or the like), use of novel additives, prescription thereof or the like.